MPI MPI Message Passing Interface Specification of message

MPI • MPI = Message Passing Interface • Specification of message passing libraries for developers and users – Not a library by itself, but specifies what such a library should be – Specifies application programming interface (API) for such libraries – Many libraries implement such APIs on different platforms – MPI libraries • Goal: provide a standard for writing message passing programs – Portable, efficient, flexible • Language binding: C, C++, FORTRAN, Python programs

General MPI Programs #include <mpi. h> main( int argc, char** argv ) { MPI_Init( &argc, &argv ); /* main part of the program */ /* Use MPI function call depend on your data partitioning and the parallelization architecture */ } MPI_Finalize();

MPI Basics • MPI’s pre-defined constants, function prototypes, etc. , are included in a header file. This file must be included in your code wherever MPI function calls appear (in “main” and in user subroutines/functions) : – #include “mpi. h” – #include “mpi++. h” * – include “mpif. h” for C codes for C++ codes for f 77 and f 9 x codes • MPI_Init must be the first MPI function called • Terminates MPI by calling MPI_Finalize • These two functions must only be called once in user code.

MPI Basics • C is case-sensitive language. MPI function names always begin with “MPI_”, followed by specific name with leading character capitalized, e. g. , MPI_Comm_rank. MPI predefined constant variables are expressed in upper case characters, e. g. , MPI_COMM_WORLD.

Basic MPI Datatypes MPI datatype MPI_CHAR MPI_SIGNED_CHAR MPI_UNSIGNED_CHAR MPI_SHORT MPI_UNSIGNED_SHORT MPI_INT MPI_UNSIGNED MPI_LONG MPI_UNSIGNED_LONG MPI_FLOAT MPI_DOUBLE MPI_LONG_DOUBLE C datatype signed char unsigned char signed short unsigned short signed int unsigned int signed long unsigned long float double long double

MPI is Simple • Many parallel programs can be written using just these six functions. – MPI_Init – MPI_Finalize – MPI_Comm_size – MPI_Comm_rank – MPI_Send – MPI_Recv

Initialization • Initialization: MPI_Init() initializes MPI environment – Must be called before any other MPI routine (so put it at the beginning of code) – Can be called only once; subsequent calls are erroneous. MPI_Init(int *argc, char **argv)

Termination • MPI_Finalize() cleans up MPI environment – Must be called before exits. – No other MPI routines can be called after this call, even MPI_Init()

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Example 1 Compile • Type “cd example 1”, you will enter the folder example 1 • Compile the MPI program by typing “mpicc -o hello. c”, and you will have the executable file hello in the folder example 1 • Type “ls”, we can check the files and folders in the current folder example 1

Example 1 Run vi mpi. sh #!/bin/bash #BATCH --job-name slurm_mpi #SBATCH --nodes 2 #SBATCH --ntasks 4 #SBATCH --ntasks-per-node 16 #SBATCH --mem 20000 #SBATCH --partition partedu cd $SLURM_SUBMIT_DIR echo 0 > cap mpirun -np 4. /hello Number of used processors

Run a Job on HPC using Slurm Common commands: sbatch: submit a job scancel: cancel a job squeue: display state of jobs